In vision, light triggers the isomerization of 11-cis retinaldehyde in rhodopsin and the cone pigments of the retina to all-trans retinaldehyde, which is then reduced to retinol in the rod outer segment and ultimately stored as retinyl ester in lipid globules of the pigment epithelium. During dark adaptation, the following reverse events occur, not necessarily in sequence: 1) transfer of vitamin A back to the rod outer segment, 2) hydrolysis and reduction of retinyl ester through retinol to retinaldehyde, 3) isomerization of all-trans to the 11-cis isomer, and 4) recombination of 11-cis retinaldehyde with opsin. Of these processes, the mode of intercellular transfer and the nature and specificity of the all-trans yields 11-cis isomerase are particularly obscure. The objectives of this study consequently are: 1) to characterize and purify retinal isomerase of the eye, 2) to define a possible linkage between the isomerization reaction and available energy donating systems, and 3) to determine the mode of intercellular transfer of vitamin A from the pigment epithelium into the rod outer segment. The working hypothesis is that all-trans retinol, in conjunction with a specific binding protein, is isomerized to 11-cis retinol by means of an energy linked reaction in the pigment epithelium, transferred to the rod outer segment in a specific complex with protein and then oxidized in the rod outer segment to 11-cis retinaldehyde.